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One of the A320 pilots told me that there is an icao rule that air should be replaced every 90 seconds? Is this true ? can someone give me a supporting article for the same

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  • $\begingroup$ Note that the fresh air inflow is mixed with recirculated air (probably the majority of the flow), and that mixed flow should achieve the entire cabin volume replacement over that time (or whatever the requirement is). I don't think it means it's all replaced with fresh air every 90 sec. $\endgroup$
    – John K
    Jan 28, 2021 at 2:39
  • $\begingroup$ Ok John still is there any figure for the same ? like I saw a video in which boeing says its done every 2-3 minutes $\endgroup$ Jan 28, 2021 at 2:46
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    $\begingroup$ There will be something in FAR25 that specifies minimum air replacement rates. ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11 I would go searching there. $\endgroup$
    – John K
    Jan 28, 2021 at 2:54
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    $\begingroup$ A typical cabin is a volume of 150 m3, and air is not replaced but diluted with new air from the outside (bleed air for the A320 - more), while some air is rejected overboard at some rate by the outflow valves to stick to the cabin pressure ("altitude") schedule. Knowing the outflow valve rate is the key for any dilution computation. However air is filtered when recirculated if this is your point. ICAO doesn't deal with cabins, but FAA, EASA, etc do (+IATA as union). $\endgroup$
    – mins
    Feb 11, 2021 at 16:28

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Let's do some simplified arithmetic:

  • The packs airflow in an A320 is 1.2 kg/s per pack in HIGH setting, but let's take the normal setting 1 kg/s, thus 2 kg/s considering both packs
  • The cabin of an A320 is around 190 m^3
  • Let's assume the cabin altitude in cruise 8000 ft, i.e. the air density at 20°C is 0.897 kg/m^3
  • The total airmass of the cabin is then 190×0.897 = 170.43 kg
  • Assuming that the cabin pressure is constant in cruise (so all the air coming from packs is going out via the outflow valve), in 90 seconds we change the following mass of air: 2 kg/s * 90 s = 180 kg

So 170.43 ~= 180, then 90 seconds is a correct estimation.

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